There are several prior art techniques for measuring the performance of a receiver signal processing system for a wireless link.
FIG. 1 shows a prior art signal processing system for a wireless transmitter 11 and receiver 21. The transmitter 11 includes a digital transmit device under test (TX-DUT) 10 which performs the encoding and modulation of data to be transmitted, inclusive of all digital functions of the wireless transmitter up to the generation of quadrature digital data for transmission 13, as is ordinarily performed in wireless signal processing of the prior art, such as the wireless LAN protocols of IEEE 802.11a, 802.11b, 802.11g, or any of the prior art wireless transmission systems. The TX-DUT 10 delivers quadrature baseband data 13 to a digital to analog converter (DAC) 12, which accepts a quadrature digital data stream suitable for modulation to a carrier frequency. The transmit RF function 14 sums the baseband analog signals from the DAC 12, modulates them to a carrier frequency such as 2.4 Ghz or 5 Ghz, amplifies the modulated signal, and couples the amplified signal to an antenna 16. A communications channel 26 carries the transmitted signals to a receiver antenna 18, which disposes the received signal to an RF baseband converter 20, which may typically include a quadrature mixer for baseband demodulation and delivery of the I and Q channels to a pair of analog to digital converters 22 which output quadrature or single ended digital data 28, which is then applied to a digital receiver signal processor RX-DUT 24. RX-DUT 24 performs the digital operations of demodulating and decoding the data stream into received data. The functional block separations shown are according to type of processing performed, and include digital processors 10 and 24, D/A and A/D converters 12 & 22, and analog RF functions 14 & 20. There are many different systems of encoding/decoding, modulation/demodulation, and these systems form the basis for the various standards of the IEEE LAN group, and many other standards-based and non-standards based communications systems.
When a particular TX-DUT or RX-DUT is to be tested, the communications channel 26 becomes an integral part of the system test result. For example, an RX-DUT 24 for 802.11b may include a sophisticated rake receiver which is effective in reducing the effects of multi-path reflections in communications channel 26, but this performance improvement may not be observed if the noise performance of the RF baseband converter 20 is poor, the incoming signal is weak, or if the communications channel 26 includes time-dependant phase shifts which are not tracked by the phase rotation correction function of the RX-DUT 24. The performance of the RX-DUT 24 is thereby limited by the performance of the systems surrounding it, and the interactions of these may become very difficult to separate and isolate when analyzing the performance of RX-DUT 24 or TX-DUT 10. Additionally, tests of the RX-DUT 24 and TX-DUT 10 in the field may produce different results due to the particular effects present at the time a particular test was performed. It is desired to conduct performance evaluations of the TX-DUT 10 and RX-DUT 24 in such a way as to include the effects of the DAC 12, transmit RF functions 14, communications channel 26, RF baseband converter 20, and ADCs 22 in a reliable, repeatable manner. It is further desired to be able to simulate the effect of a single change in performance of DAC 12, transmit RF functions 14, communications channel 26, receive RF functions 20 and ADC 22 on the TX-DUT and RX-DUT.
U.S. Pat. No. 6,308,064 by Green describes a testing system for interconnecting wireless systems to a plurality of antennas placed with a variety of separations and reflective structures, where each of the antennas are individually selectable as part of the testing methodology. In this manner, the communications channel may be modeled through the placement of antennas.
U.S. Pat. No. 6,542,538 by Fischel et al describes a method for testing a wireless link by transmitting a pseudo-random number sequence across the link, whereby the receiver synchronizes to the pseudo-random link to test for received errors.
U.S. Pat. No. 6,571,082 by Rahman et al describes a test simulator for modeling the effects of multi-path, attenuation, and doppler shift on a signal, where the test simulator is placed in the wireless RF receive path of a receive signal processor used in a wireless link.
U.S. Pat. No. 6,724,730 by Mlinarsky et al describes a test system whereby the transmitter and receiver are coupled with an RF link, and the simulator is placed in the wireless link between the transmitter signal processing and receive signal processing, where the simulator includes variable attenuation, and a multi-path and doppler simulator.
U.S. patent application Ser. No. 2003/0236089 describes a test system for cellular wireless systems, whereby the received signal is converted to baseband prior to sequential processing by modules which provide channel simulation for use to a plurality of devices under test.